1. Field of the Invention
The present invention relates to an image forming apparatus such as an electronic copying machine.
2. Description of the Related Art
In an electronic copying machine using an electrophotographic process, for example, an original set on an original table is concealed under an original cover. The original and the cover are illuminated by means of an exposure lamp. Light beams reflected by the original and the cover are guided onto a photosensitive drum. As a result, an electrostatic latent image corresponding to an image of the original is formed on the surface of the previously charged drum, and any undesired electric charge around the latent image is removed.
If the original is scanned uncovered, for example, light beams applied to regions outside the original cannot be reflected by the original cover, so that they cannot be led to the photosensitive drum. Thus, the undesired charge around the electrostatic latent image corresponding to the original image cannot be removed, so that a toner adheres also to the peripheral regions outside the latent image, during a developing process. Therefore, if the original is smaller in size than a copying sheet, or if the size of the formed image is smaller than the sheet size although the original and the sheet are equal in size, as in the case of reduced-scale copy, the peripheral region of an output image (copy image) around the region corresponding to the original image is blackened by the toner. When copying a thick original such as a book, moreover, the original cover cannot be fully in contact with the original table, so that an undesired black image develops on the peripheral region of the output image.
If the undesired black image (black solid image) develops in this manner, the output image is unpleasant in appearance and poor in quality. Further, a waste of the toner entails an increase of the consumption of a developing agent.